Conventionally, electronic devices such as a sensor for measuring an air pressure (internal pressure) of the compressed air that fills up a tyre and a transmitter for transmitting radio waves containing information measured by that sensor to a wheel side have been arranged inside a tyre, or more specifically, in a space partitioned by a tyre and a rim wheel (a partitioned space, hereafter).
In FIG. 1 of Japanese Patent Application Laid Open No. 2004-148953, an electronic device 7 is arranged between an inner liner 6 positioned inside a belt layer in a radial direction of a tyre 1 and that belt layer.
In Japanese Patent Application Laid Open No. 2003-146033, an air valve 21 attached to a rim wheel 22 and an electronic device 10 are set in an integrated form such that a pressure or the like of the compressed air that fills up a tyre can be measured. When FIG. 1 is combined with FIG. 2 and FIG. 3, an antenna 14 is positioned inside the partitioned space.
As a result of arranging the electronic device in the partitioned space as described above, the radio waves transmitted by that electronic device will be attenuated by repeating reflections in that partitioned space. For this reason, there has been a problem that the radio waves emitted outside the tyre become weak compared with the radio waves transmitted by the electronic device.
More specifically, in a tyre, various metallic materials (wire cords and steel cords, for example) are used for portions (a bead portion and a carcass layer, for example) constituting that tyre, and in a rim wheel, iron, aluminum or the like is used, so that the radio waves transmitted by the electronic device will be attenuated by repeating reflections in the partitioned space.
Also, in FIG. 2 of Japanese Patent Application Laid Open No. H 11-240315, an antenna 16 is directly projected outside at a position close from an air control valve 10, rather than piercing the antenna 16 through the air control valve 10. In such a structure, an LC resonance (L: coil, C: capacitor) will be generated, so that there arises a need to adjust a circuit for matching the antenna 16 (such as a resistance capacitor and a coil).
In addition, in a tyre air pressure warning device of FIG. 1 of Japanese Patent No. 3392364, insulative grommets 19, 20 are disclosed. However, these insulative grommets 19, 20 are insufficient for high frequencies, so that the LC resonance cannot be prevented completely.